Electrostatic pinning of extruded polymeric film

ABSTRACT

In apparatus for application of an electrostatic charge to cause a dielectric web to adhere to a grounded roll, the improvement comprising movable insulating tubes over the terminal supports and the electrode wire.

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POLYMERIC FILM 4 Claims, 3 Drawing Figs. [52] 8/ ABSTRACT: In apparatus for application of an electrostatic [5!] Int. Cl....... 329d 7/20, charge to cause a dielectric web to adhere to a grounded roll, 329d 7/ 2 the improvement comprising movable insulating tubes over the terminal supports and the electrode wire.

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PATENTED HAR23I97I 3,157 1,853

INVENTORS MELTON O. DENTON SAMUEL E. MEGGS A'ITO EY ZELECTLQUSTATHQC IPHNNWG Oil EXTRUDED lUlLYIl/llERIlQ l lilhld BACKGRQUND OF THE lNVENTlON in the casting of molten, crystallizable thermoplastic web, it is necessary. to quickly cool the molten web below the glass transition temperature to minimize crystallization. it is believed that excessive crystallization in the web interferes with orientation, causing areas of haze and gauge irregularity in the oriented product. The extruded web is generally cooled by casting the molten thermoplastic material onto a chilled, moving surface, usually a rotating drum. Previous attempts to increase the speed of this procedure for more eiiicient and economical operation have resulted in poor gauge uniformity and regularly recurring haze patterns known in the art as venetian blind haze. in addition, increased speed often results in neck-in," a lateral shrinkage in the quenched film. To correct these problems, many techniques have been employed to force the web into closer contact with the quench drum.

One of the most useful methods of pinning the molten web to the quenching surface involves imparting an electrostatic charge to the film across the full width of the film. This method is described, for example, in Owens et al., U.S. Pat. No. 3,223,757, hereby incorporated by reference. Unfortunately, however, still higher casting speeds result in a decrease in the ability of a given electrostatic force to pin the web to the quenching surface, due to the increasing quantities of air that are pumped in and between the web and the drum. The layer of air trapped between web and drum prevents the effective transfer of heat to the drum, and thereby reduces the effectiveness of the quenching operation. Further, the appearance of pinner bubbles at the interface of the web and drum results in unacceptable surface irregularities in the finished film product.

Attempts have previously been made to increase the electrostatic force generated by the pinning wire by increasing the voltage. These attempts, however, have been ineffective, since increased voltage generally causes a catastrophic electrical breakdown between the electrode and the drum before a sufficient charge can be applied to the web to effect any substantial increase in the pinning force. The sparking momentarily interrupts the electrostatic field of the electrode which contributes to the pinning force.

SUMMARY OF THE lNVENTlON The instant invention provides an improved apparatus for the full sheet electrostatic pinning of a polymeric web to a quenching surface which not only permits the use of increased voltages to achieve a greater pinning force, but additionally is adjustable with respect to the width of the electrostatic field produced.

Specifically, the instant invention provides an improved apparatus for electrostatically pinning a molten web to a quenching surface comprising a wire electrode connected to a high voltage source and supported between lateral supports, the wire electrode positioned above the approximate touchdown point of the web onto the quenching surface, and the width of the web being less than the width of the quenching surface, the improvement wherein the lateral supports comprise electrically insulated conductive rods extending toward the quenching surface, the rods being substantially circular in cross section and terminating with a section extending inwardly and parallel to the quenching the wire electrode joined to the end of the sections of the supporting rods parallel to the quenching surface, the improvement further comprising axially movable sleeves of dielectric material positioned around the junctions of the wire electrode and the supporting rods, the sleeves extending outwardly along the wire and rods from the edges of the web. BRIEF DESCRIPTION OF THE DRAWINGS HG. ii is a cross-sectional view of one embodiment of the electrode support arrangement of the instant invention.

H6. 2) illustrates an apparatus of the instant invention in conjunction with a casting and quenching apparatus.

FIG. 3 is a plan view of the apparatus of H6. 2. DESCRlP- TlON OF THE. PREFERRED EMBODlMENTS The instant invention can be readily understood by reference to the drawings, in which the same numerals indicate like elements in the three figures.

For purposes of the instant disclosure, the terms inward" and outward refer to directions along a line parallel to the plane of the quenching surface and perpendicular to its edge, inward and outward respectively indicating a direction toward the middle or edges of the surface.

The electrode support is shown in its basic form in H6. 1. Rod ill having insulating layer 111 is provided with holes 12 on the inward extremity through which electrode wire i3 is looped and secured. The rod can be made of any conductive material such as brass, copper, stainless steel and the like and should be of sufficient thickness to support the electrode wire under the desired tension. A one-eighth inch brass 3,73lhas been found to be particularly satisfactory for many applications of the instant invention.

The rod llll is coupled to a high voltage sourceinot shown) by high voltage connector 1% such as that commercially available from Gremar Manufacturing Co. of Wakefield, Massachusetts, as Model No. 8,731. The conductor is soldered to the end of rod 10 and engages a mating part coupled to the high voltage source. Electrode rod 10 fits snugly in insulator 15, and is held in place on the adjacent framework by female threaded sleeve l6, which engages a threaded male portion (not shown).

An essential element of the present invention is slideable insulating sleeve 25 which fits snugly over rod ll) and, with plug 26, engages wire 13 which passes through a small hole in the plug. This sleeve provides means for adjustment of the effective width of the electrode. Materials which can be used for the sleeve include any dielectric that can be formed into a substantially rigid cylinder, such as hard rubbers and resins, e.g., fluorocarbon resins, urea formaldehyde resins, filled phenol formaldehyde resins, nylons and cast epoxy resins. it has been found that Teflon" fluorocarbon resin is particularly well suited for this application, since it is stable at temperatures encountered in the vicinity of the casting hopper, and has excellent insulating properties. The size of the sleeve will, of course, vary with the size of the supportingrod ill, but, for a rod having a diameter of one-eight inch a sleeve having an inner diameter of about one-eighth inch and an outer diameter of about three-sixteenth inch can be used. The length of the sleeve will vary according to the distance normally encountered between the edge of the extruded film and the end of the support rod to which the wire is attached.

H68. 2 and 3 illustrate an apparatus of the present invention mounted on melt extrusion equipment. In the drawings, hopper l7 extrudes molten flat web 18 onto metal quench drum l9. Mounted on frame 20 are electrode supports 21 and 22, which support wire 13 in a manner illustrated in FIG. 1 to maintain wire 13 in a taut condition as it spans roll 19 and web 18. Supports 2i and 22 are electrically coupled to high voltage source 23 which has one side grounded to provide an electrical circuit through roll 19, which is provided with ground 26. It should be observed that quench roll 19 is wider than web it so that bare metal would be exposed to the electrode wire 13 except for insulating sleeves 255 which permit the adjustment of the effective length of the electrode.

FlG. 3 is a view from above, omitting the extrusion apparatus in order to better illustrate the invention. The effective length of electrode wire 13 is limited by sleeves 2 to be coextensive with web l8, and the sleeves extend outwardly to cover the junction of the wire and rod.

The invention is further illustrated by the following examples.

EXAMPLE 1 A polyethylene terephthalate film is extruded and electrostatically pinned onto a quench drum using apparatus similar to that illustrated in FIG. 1 of Owens et al., US. Pat. 3,223,757. The thickness of the film after biaxial orientation is 0.92 mil. The voltage on the pinning apparatus is adjusted to the maximum obtainable without electric breakdown, and the maximum production rate in lbs/hr. at which consistently high quality film can be produced is determined and designated as R.

Pinning apparatus as described in the above specification is substituted and the voltage adjusted to the maximum obtainable without breakdown. The apparatus comprises one-eighth inch brass supporting rods and Teflon" fluorocarbon resin sleeves. The maximum production rate of the 0.92 mil film obtained using the pinning apparatus of the instant invention is 1.23R.

EXAMPLE 2 Example 1 is repeated, except that the original thickness of the web is changed to yield a 1.42 mil film after orientation, and the maximum production rate obtainable using the Owens et al. apparatus is designated as R. The maximum production rate obtained using equipment of the instant invention is l.45R'.We claim:

1. In an apparatus for electrostatically pinning a molten web to a quenching surface comprising a wire electrode connected to a high voltage source and supported between lateral supports, the wire electrode positioned above the approximate touchdown point of the web onto the quenching surface, and the width of the web being less than the quenching surface, the improvement wherein the lateral supports comprise electrically insulated conductive rods extending toward the quenching surface, the rods being substantially circular in cross section and terminating with a section extending inwardly and parallel to the quenching surface, the wire electrode joined to the ends of the sections of the supporting rods parallel to the quenching surface, the improvement further comprising axially movable sleeves of dielectric material positioned around the junctions of the wire electrode and the supporting rods, the sleeve extending outwardly along the wire and rods from the edge of the web.

2. An apparatus of claim 1 wherein the sleeve consists essentially of fluorocarbon resin.

3. An apparatus of claim 1 wherein the sleeve consists essentially of a hard rubber.

4. An apparatus of claim 1 wherein the rods consist essentially of brass and have a diameter of about one-eighth inch and wherein the rods are electrically insulated by an outer layer of fluorocarbon resin. 

2. An apparatus of claim 1 wherein the sleeve consists essentially of fluorocarbon resin.
 3. An apparatus of claim 1 wherein the sleeve consists essentially of a hard rubber.
 4. An apparatus of claim 1 wherein the rods consist essentially of brass and have a diameter of about one-eighth inch and wherein the rods are electrically insulated by an outer layer of fluorocarbon resin. 